This invention relates in general to foam or foam core structural panels and, more specifically, to structural panels having a polyimide foam core of controlled density.
Foamed plastic structures have long been made using a variety of synthetic resins and various molding methods. In some cases blowing agents are added to cause foaming when heated and in others the polyimerizing or curing reaction generates the foaming agent. In some cases open-cell foams are produced and in others, closed-cell foams result. Depending on the resin to be used and the required physical characteristics of the product these different prior methods may or may not be acceptable.
For many applications, such as airplane or ship ducting, in manned space vehicles or other human-occupied closed structures, panels using polyimide foam have been found to be ideal since polyimides are highly fire resistant and do not give off toxic gases when heated to degradation temperatures. Polyimides, however, require high curing temperatures which many of the prior foam manufacturing and reshaping methods cannot efficiently apply. Also, for many applications the density of the foam (which directly affects the weight rigidity and strength of the foam structure) must not only be closely controlled but also be variable within wide limits depending upon the planned use for the foam structure being made.
Foaming between moving endless belts has been used with a variety of resins. This method has been used, for example, with phenolics, as described by Bruning et al. in U.S. Pat. No. 3,883,010, with polystyrene as described by Charpentier in U.S. Pat. No. 3,863,908 and with polyurethane as described by Willy in U.S. Pat. No. 3,998,884. While apparently effective with those resins, this method is not applicable to controlled density polyimide foam because of the much longer periods at much higher temperatures required for polyimides which require excessively long and/or very slowly moving belts operating in a very large high temperature oven. Also, this technique produces only flat panels and not the other shapes often required.
Polyimide foams, therefore, are ordinarily produced in closed molds where particular thicknesses and surface configurations are required. Typical of such methods are those disclosed by Long et al. in U.S. Pat. No. 4,621,015 and Shulman et al. in U.S. Pat. No. 4,647,587. If desired, a liner may be placed in the mold prior to introduction of the foamable liquid to bond to the foam as it expands and cures. The foam produced in closed molds tends to be very irregular and lack uniformity of cell size, density and strength across the mold. Often, the gasses emitted by the polyimide precursor during foaming inhibit the growth of the cells in the foam itself, making foaming action low and unpredictable. These problems increase when attempts are made to vary the foam product density by varying the amount of precursor placed in the mold.
Impervious face sheets are required on many foam structures or panels. Face sheets can be adhesively bonded to the foam panels after the panels are cured, or the face sheets can be placed in the mold prior to foaming. Adhesive bonding has the inherent disadvantages of adding another agent which usually will not have the high temperature resistance of the foam and may emit toxic gasses when heated. The bonding step adds complexity to the manufacturing operation and, if not very carefully done, may result in bond failure and de-lamination in use. Where the face sheets are placed in the mold prior to foaming poor bonds often result because the foaming resin does not adequately wet the face sheets. Excess foaming material may be required, resulting in higher foam weight, in order to assure complete mold filling and sufficient foam pressure against the face sheet during foaming to obtain a good bond.
In some cases, polyimide precursors are foamed under ambient pressure in open molds. The resulting foam layer is then sliced parallel to the bottom of the mold to produce panels of the desired thickness. Ordinarily, only flat panels can be made by this method. While this produces more uniform cell size and density, it is difficult to produce panels of different, controlled, varying densities for different or complex structural application. Also, adhesive bonding, with all of the drawbacks mentioned above, is the only available method for applying a face sheet to the sliced surface of the foam panel.
Thus, it is apparent that there is a continuing need for a method of making complex polyimide foam structures having uniform cell size, varying density and other physical characteristics while permitting easy and precise control of foam density and which allows face sheets to be securely and reliably bonded to one or more foam surfaces.